AGE OF THE CHICXULUB IMPACT AND MASS EXTINCTION, BRAZOS RIVER, TEXAS, USA

Multidisciplinary investigations of upper Maastrichtian to lower Danian sequences along 3.5 km of the Brazos River in Falls County, Texas, reveal the stratigraphic separation and sequential depositional history of three events : 1) the primary Chicxulub ejecta layer, 2) the sandstone complex with reworked impact spherules, and 3) the Cretaceous-Tertiary boundary (KTB) mass extinction. The sandstone complex with reworked impact spherules is the most prominent outcrop feature and infills a channel, or incised valley that formed during the latest Maastrichtian sea-level fall about 100-150 ky prior to the KT boundary. Basal channel deposits consist of coarse shelly glauconitic sand with large lithified clasts that contain impact spherules and large bored and encrusted phosphatized concretions. These spherule-rich clasts bear witness to erosion of the older primary Chicxulub spherule ejecta layer, which was lithified prior to erosion and redeposition at the base of the channel. The primary Chicxulub ejecta layer was discovered 40–65 cm below the unconformity at the base of the sandstone complex in a 3-cm-thick yellow clay layer that consists of cheto smectite (altered impact glass) interbedded in claystones of the Corsicana Formation. Deposition of the primary Chicxulub ejecta layer is estimated at about 200 ky prior to the KTB. The KTB mass extinction and characteristic δ¹³C shift occur up to 1 m above the sandstone complex during a rising sea level. The temporal and stratigraphic separation of these three events together with the faunal, geochemical and mineralogical data unequivocally places the Chicxulub impact in the latest Maastrichtian predating the KTB mass extinction. No significant faunal changes and no species extinctions coincide with the primary Chicxulub impact ejecta layer.